1. Field of Invention
The present invention relates to a method of manufacturing integrated circuits. More particularly, the present invention relates to a method for forming a multi-layered liner on the sidewalls of a node contact opening.
2. Description of Related Art
As semiconductor fabrication moves towards the manufacture of devices having line width under 0.25.mu.m, width of a word line, a bit line and a contact node opening as well as their distances of separation from each other in DRAM must be reduced according to the design rules. Due to a shorter distance between neighboring bit lines, alignment accuracy of a node contact opening is worse than before. To prevent too much parasitic capacitance between the bit line and the node contact due to lower alignment accuracy, an insulating liner is normally formed on the sidewalls of the node contact opening. In fact, the liner is able to provide the necessary insulation between the subsequently formed node contact and the bit line.
In general, the liner is either a silicon oxide layer or a silicon nitride layer. FIG. 1 is a schematic, cross-sectional view showing a conventional node contact opening having a liner layer. As shown in FIG. 1, the node contact opening structure is built upon a substrate 100 (for simplicity, devices within the substrate 100 are not drawn). The structure includes two dielectric layers 108 and 110 above the substrate 100, bit lines 106 embedded within the dielectric layer 110, a node contact opening 102 passing through the dielectric layers 110 and 108 to expose a portion of the substrate 100, and a liner layer 104 on the sidewalls of the node contact opening 102. The liner layer 104 is formed by depositing a conformal insulating layer (not shown) over the dielectric layer 110 as well as the interior sidewalls and bottom of the node contact opening 102. Next, the insulating layer is etched back to remove the insulating layer above the dielectric layer 110 and the insulating layer at the bottom 102a of the node contact opening 102. The remaining insulating layer on the sidewalls of the node contact opening 102 after the etching operation becomes the liner layer 104.
Although silicon oxide liner 104 can provide good insulation, it can be easily damaged by diluted hydrofluoric acid. Diluted hydrofluoric acid is used for removing native oxide layer above the substrate before the node contact is formed. Therefore, a portion of the silicon oxide of the liner layer 104 may be etched away.
Normally a thicker oxide liner layer is formed to prevent the removal of too much silicon oxide from the liner layer 104 by hydrofluoric acid. However, by so doing, a thicker layer of oxide is also laid over the bottom part 102a of the node contact opening 102. Therefore, time required to etch away the entire oxide layer at the bottom 102a of the node contact opening 102 is longer. A longer etching time not only leads to the removal of extra oxide from the top portion 102b of the node contact opening 102, but also leads to the removal of oxide on the sidewall, as well.
Alternatively. silicon nitride can also be used to form the liner layer 104. Although silicon nitride liner is more resistant to the attack by hydrofluoric acid than silicon oxide liner, silicon nitride tends to have more pinholes. Therefore, the insulation capacity of a silicon nitride layer is inferior. Often, this leads to larger leakage current between the node contact and the bit line 106 resulting in electrical instability problems.
In light of the foregoing, there is a need to improve the liner layer on the sidewalls of a node contact opening.